Recombinant mycobacterial antigens have been tested for T-cell reactivity and specificity in an effort to develop a specific skin test reagent that will distinguish M. tuberculosis infections from those caused by the M. avium-M. intracellulare complex (MAC). Recombinant antigens isolated from M. intracellulare (4 antigens) and M. avium (2 antigens) lgt11 expression libraries were purified by HPLC molecular exclusion chromatography and tested for delayed type hypersensitivity reactions in guinea pigs sensitized to various mycobacterial species. All six antigens elicited reactions but none was specific. Several additional antigens are currently being purified. Since a specific reagent will probably require use of individual T-cell epitopes rather than whole antigens, T-cell epitopes are also being identified and analyzed. Peptides inferred from the nucleotide sequence of one M. intracellulare antigen have been synthesized, purified by reversed phase HPLC and analyzed for immunological activity. These peptides react specifically with the monoclonal antibody used to isolate the original antigen in ELISA assays and inhibit binding of the antibody to M. intracellulare sonic extracts. Two of these peptides exhibit delayed type hypersensitivity reactions in guinea pigs but neither is specific. Additional peptides are now being identified and synthesized.